1. Field of the Invention
The present invention relates to a demodulator circuit used in a tag or transponder in a radio frequency identification (RFID) system, and more particularly, to a demodulator circuit capable of generating a clearly level-identified output signal upon receiving an input amplitude signal having several modulation depths.
2. Discussion of Related Art
In general, an RFID tag is used in wireless communication systems such as an RFID system, and operates by receiving an amplitude signal from a reader. A communication distance between an RFID tag and a reader is defined by power consumption of the RFID tag. Since such a communication distance is a very important factor in application fields as well as performance of a system, researches for improving the communication distance have been constantly in progress.
However, conventional RFID tags require extremely-limited available driving voltage, and amplitude signals input thereto have very low power levels. Thus, it is exceedingly difficult to embody a data recovery circuit capable of detecting a modulation signal at low power.
Methods for solving such a technical problem have been suggested in U.S. Patent Laid-Open Publication Nos. 20040056691A1 and 20050104573A1 in which a voltage multiplier for data recovery comprises two output capacitors having different capacitance connected to inputs of a voltage comparator, and the voltage comparator compares voltages of the two capacitors with each other whenever the level of a tag input signal is changed. In addition, a method for comparing an output voltage of a voltage multiplier for data recovery with a reference voltage has been suggested in U.S. Patent Laid-Open Publication No. 20020149482A1, and also a method for converting output voltage of a voltage multiplier for data recovery into current and recovering data from variation of the current has been suggested.
However, the above-mentioned methods have drawbacks in that the period of a signal to be recovered may be somewhat changed by an offset difference of a compared voltage due to use of a comparator, there are many current paths due to the same reason, and a large amount of power is consumed due to use of an inverter.
When a reader used in an RFID system outputs an amplitude signal, a tag converts the signal into a direct current (DC) signal and then uses the DC signal as driving power for an entire circuit in the tag while detecting amplitude variation of the signal and decoding command data. A small modulation depth of an input amplitude signal is favorable to obtain large driving power, but a large modulation depth is favorable to easily detect an envelope upon demodulation. Here, a modulation depth is defined by following Formula 1:Modulation depth=(high−low)/(high+low)×100%.  Formula 1
Because of the above-described reason, most RFID systems comprise a demodulator circuit that utilizes 100% of the modulation depth in the relatively short duration of a low level by adopting an encoding method making the duration of a high level of an input amplitude signal longer than the duration of a low level. Here, a conventional RFID demodulator circuit compares a signal passing through an envelope detector using a comparator, and generates a detected signal that can be classified into a high level and a low level. However, when the reading range of an RFID system increases, it is hard to utilize 100% of a modulation depth, and only 10 to 30% of the modulation depth may be used.
When only a small modulation depth is used, the level of a data signal to be recovered by a tag does not have a large difference from the level of an amplitude signal for supplying driving power. The demodulator circuit of the tag should be able to recover small variation of the input signal into a logic signal of “0” or “1.” However, when the input signal is slightly changed, an amplifier should be additionally installed or the number of stages of a voltage multiplier should be increased so as to amplify the signal. Therefore, power consumption for data recovery increases, and desired impedance of a tag is not easily obtained or driving power for the tag needs to be reduced.